The aortic and pulmonary valves, collectively known as arterial valves, are located respectively in the left and right ventricles of the heart. They serve to prevent regurgitation of blood from the aortic artery or pulmonary artery into its associated ventricle when that ventricle is in its expanded state. Both valves consist of three semicircular leaflets or flaps attached by their convex margins to the wall of the artery at its junction into the ventricle. In both the aortic and pulmonary valves, the straight border of each leaflet is free and directed upward into the artery.
Although the aortic (left ventricle) valve is larger, thicker and stronger than the pulmonary (right ventricle) valve, the openings of both arteries into their ventricles are generally circular, and form an annulus or ring that is composed of fibrous rather than muscular tissue. Another characteristic shared by these valves is the presence of pouches or sinuses one behind each leaflet, that exist between the valve and the wall of the artery. The blood, in its regurgitation back toward the ventricle, finds its way into these sinuses, and so closes the valve-flaps.
Healthy functioning valves such as, for example, the aortic valve require a secure meeting of the leaflet free borders along the lines at which they come together. When these free borders securely meet so that no blood can escape back into the ventricle, this positive closing of the leaflets is called coapting. The regions extending along and adjacent to the arterial wall at which the leaflets "coapt" are called commissures. In a properly functioning valve, the circular annulus (defining the border between the sinuses of the ascending vessel and the ventricle) provides a firm base ring for the convex margins or "cusps" of the leaflets so that their free borders can meet securely. A common defect leading to aortic valve dysfunction is a dilation or stretching of this arterial annulus, and often its associated valve sinuses, preventing positive closure of the attached leaflets and possibly even allowing one or more leaflets to flip over, or "prolapse", toward the ventricle. Total valve replacement is one solution to valve dysfimction, but repair of the annulus or sinuses by various techniques, thereby retaining the natural valve, is preferred.
Among such techniques for reconstructing the aortic annulus is the practice of drawing in the stretched annulus by means of sutures threaded along the dilated circumference of the annulus. The success of such a technique varies significantly with the skill of the surgeon, and the technique may produce inconsistent circumferential pleats, which would adversely affect desirable annular symmetry. An alternative to the all-suture technique of annular reconstruction is the use of a prosthesis to restore the normal circumference of the annulus. Correction by a prosthesis is potentially longer-lasting than is the all-suture technique, since after the latter procedure, tissues will tend to compensate against abnormal pressure by the sutures, possibly defeating the purpose of the repair. A prosthesis has the additional advantages of providing the predictability of a defined structure, and of enabling the surgeon to determine the ultimate outcome of the procedure before closure, i.e., without having to wait for post-operative analysis.
However, much of the prior art devoted to annular prostheses has been directed to the reconstructing of the atrioventricular mitral bileaflet and tricuspid trileaflet valves whose specific non-circular configurations are not similar to those of the aortic or pulmonary valves. Accordingly, these prior art prostheses are not suitable for the restoring of an aortic or pulmonary annulus because, unlike the circular aortic or pulmonary valve annuluses, a mitral valve or tricuspid valve annulus is decidedly D-shaped. The mitral and tricuspid annuluses include straight segments formed of dense tissue so that their arcuate portions are relatively more subject to problem elongation. Thus, for their reconstruction, these valves require certain prostheses that are typically anchored to the fibrous straight section of the annulus in order to reconfigure the elongated arcuate portion. For example, U.S. Pat. No. 3,656,185 to Carpentier discloses a mitral valve reconstruction annular prosthesis. It relies on securing its rigid portion to the mitral valve annulus straight segment, and is not compatible with the circular shape of the aortic annulus which lacks this rigid foundation upon which to anchor the prosthesis. Moreover, such rigid attachment to the annulus tends to detrimentally inhibit the natural movement of the annulus during a cardiac cycle of expansion and contraction.
Other annular prostheses such as that described in U.S. Pat. No. 4,489,446 to Reed are also configured to attach to the unique physical aspects of the non-circular atrioventricular valve annulus. Specifically, Reed utilizes reciprocating members which are sutured to the relatively more dense muscular structure of the mitral or tricuspid annulus as contrasted with the weaker fibrous composition of the aortic annulus. In addition, the reciprocating members of Reed are configured for implantation below the valve, not for positioning within the artery as would be desirable in order to reconstruct dilated valve sinuses.
A prosthesis which does not rely on the distinctive shape of the mitral annulus, and thus may be suitable for reconstruction of an aortic annulus, is disclosed in U.S. Pat. No. 4,917,498 to Carpentier. It comprises a ring of many linearly connected segments which depends for its structural integrity on a fabric holding the chained elements together. Such a prosthesis avoids the mitral valve-specific shortcomings of the previously noted atrioventricular prostheses but, because it is configured for attachment to annulus tissue, it is similarly unsuitable for intra-arterial positioning, i.e., above the valve as opposed to below it. This severely limits its ability to reconstruct the aortic valve annulus by beneficially constricting the dilated sinuses that are disposed within the artery.
Accordingly, it is an object of this invention to provide a prosthesis specifically directed to safely, consistently and durably restoring the generally circular shape of an aortic annulus to produce positive valve closure.
It is another object to provide a prosthesis that is simple to manufacture in a range of sizes corresponding to the range of normal valve annulus diameters.
It is a further object to provide a prosthesis rigid enough to maintain a functional annular shape yet flexible enough to allow natural movement of the annulus during the cardiac cycle.
It is a still further object to provide the choice of positioning the prosthesis of this invention below the aortic valve or alternatively above the valve within the walls of the artery.